This invention relates generally to the medical arts and more particularly to a needle or other tubular cannula having improved sonographic visibility.
The present invention is particularly suited for use in amniocentesis procedures and will be described herein as an improved "amniocentesis needle". It must be appreciated, however, that the means for improving acoustical detectability of the present invention may have broad applicability and may find utility in connection with virtually any needle, tube, catheter, cannula, trocar, or object wherein improved ultrasonic visibility is desired Specific examples of devices other than amniocentesis needles wherein the acoustical improvements of the present invention may be applicable include, but are certainly not limited to, various biopsy needles, aspiration cannulae, trocars, insertable scopes, surgical instruments, drug-containing implant devices, and cardiovascular catheters of the type routinely placed by echocardiographic guidance.
Referring specifically to amniocentesis procedures, the safety and efficiency of such procedures has been substantially improved in recent years by the application of real time ultrasonic imaging as a means for monitoring the percutaneous transabdominal insertion and intrauterine placement of the aspiration needle. Such ultrasonic imaging and guidance provides a safe and non-invasive means for ensuring proper placement of the needle to avoid inadvertant aspiration of blood from the uterus or placenta or damage to the fetus The proper placement of the needle prior to aspiration is particularly important in that any contamination of the amniotic fluid sample by blood from the uterus or placenta may render the sample unacceptable for analysis, thus requiring that the entire procedure be repeated, thereby multiplying the attendant risk of injury to the mother and/or fetus. Another benefit derived from routine ultrasonic guidance has been a significant decrease in the incidence of fetal injury. Most injuries to the fetus during amniocentesis have previously occurred due to direct traumatization of the fetus by the aspiration needle. Thus, the ability to carefully monitor placement of the needle tip relative to the fetus is a key factor in avoiding injury to the fetus. Unfortunately, the acoustical properties and resultant "imageability" of the commonly used amniocentesis needles are generally less than optimal for the reasons hereinafter discussed.
Today, most aminocentesis procedures are routinely carried out using standard 20-gage "spinal" needles of the type generally used to perform lumbar puncture and aspiration of cerebrospinal fluid The use of such spinal needles to perform amniocentesis procedures is, however, associated with several deficiencies. First, the single opening at the end of the standard spinal needle is known to become easily occluded by solid materials commonly entrained within the amniotic fluid. In practice, such occlusion of the needle tip generally requires prolongation of the procedure while the attending physician endeavors to reposition the needle tip. If such attempts are unsuccessful, it may then be necessary to fully repeat the percutaneous puncture and transabdominal placement of the needle so as to repeat the aspiration of the amniotic fluid sample Second, the smooth polished surface of the standard spinal needle is generally difficult to image by ultrasonic means. This is due to the smooth shaft of the needle typically reflecting the incident ultrasound beam in a non-diffracted, unidirectional, geometric manner. Thus, unless the receiving transducer element is precisely positioned in the path of the geometrically reflected beam, such beam will avoid the transducer and thus escape visualization by the imaging apparatus.
In contrast to the smooth surface of the needle shaft, the beveled tip of the standard spinal needle is usually detected by the imaging equipment because the incident ultrasound beam tends to be diffracted, rather than simply reflected, from the roughened edge and angular disposition of the beveled needle tip. Such diffraction of the incident beam from the beveled needle tip gives rise to a diverging accoustical reflection or echo. As a result of such divergence, it is most probable that some portion of the reflected beam will arrive at the desired transducer element Such diffracted echos are, however, associated with at least one drawback in that they are generally bright "blooming" echos which frequently exhibit phantom displacement from the actual position of the needle tip.
In addition to the above-described problems stemming from the poor ultrasonic detectability of the standard spinal needle, other problems are typically associated with the ultrasonic imaging of all amniocentesis needles. Specifically, ultrasonic imaging systems which employ single-element, fixed focus transducers generally emit incident beams of fixed focal depth (i.e. short range =1-4 cm, medium range =4-8 cm, and long range =6-12 cm) In amniocentesis procedures, the desired focal depth is generally within the "medium range" as the incident beam must travel through 3 to 4 centimeters of soft tissue, i.e. skin, muscle, and fat before reaching the amniotic cavity. At such depth, the focal region is normally less than 10 mm in diameter Thus, if the shaft of the needle is not precisely positioned within such focal region, any acoustical reflection or echo emanating therefrom will be of diminished amplitude and, thus, will be difficult to image. For this reason, poor focusing of a standard smooth walled needle may tend to further complicate the problems created by the non-diffracted, geometric reflection of the incident beam from the smooth needle shaft.
More complex ultrasonic imaging systems employed today incorporate annular or linear array technology wherein multiple independent transducer elements are arranged concentrically or linearally about a central transducer element. The formation and linear focus of the incident beam are achieved by pulsing the individual transducer elements in a time delayed or phased fashion. Thereafter, the electronic signals generated by the returning echos are received and combined using similar timed delays. By such method, these phased array systems are capable of being electronically and dynamically focused in two dimensions without mechanical movement or adjustment of the transducer head. However, because such electronic focusing is generally limited to only two dimensions, the focal depth continues to be set by specific mechanical lenses placed on each element of the transducer. Thus, any disparity between the position of the needle and the preset focal depth will remain problematic, even though a dynamically focusable linear or annular phased array system is employed.
Furthermore, linear phased array imaging systems are known to produce spurious echos which may interfere with visualization of the amniocentesis needle. These spurious echos, when passing through tissue, are of generally low amplitude and may be easily differentiated from anatomical structures. However, amniotic fluid differs from soft tissue structures in that it tends to "fill in" with numerous high amplitude spurious echos The transmission of such high amplitude spurious echos within the fluid further complicates the desired visualization of the needle while it is positioned within the amniotic cavity.
Thus, because of the poor echogenicity of the standard smooth-walled spinal needle, in conjunction with the various focusing problems inherent in all ultrasonically guided amniocentesis procedures, there exists a present need in the art for an improved amniocentesis needle which is readily visible by sonographic means.
Previous attempts have been made to improve the sonographic visibility or echogenicity of certain amniocentesis needles; however, such efforts have failed to produce a truly optimal amniocentesis needle. As a result, the medical profession has largely failed to adopt the marginally "improved" amniocentesis needles, opting instead to continue using the standard smooth-walled spinal needle. In fact, it is estimated that more than ninety percent of the amniocentesis procedures currently performed in the U.S. continue to employ standard 20 or 22 gage spinal needles, despite their poor imageability.
Most of the prior art attempts to improve the sonographic visibility of smooth walled spinal needles have involved roughening or texturing the outer surface of the needle in order to promote some diffraction of the echoing beam. Specifically, it has been found that sonographic visibility of the needle shaft may be improved by roughening or scoring the outer surface of the needle itself or by roughening the surface of a solid stylet which is disposed axially within the lumen of the needle. While such roughening or scoring of the needle/stylet may somewhat improve the ultrasonic detectability of the needle by causing diffraction of the echoing beam, the process of scoring or roughening the needle surface further adds to the cost of manufacturing. Also, it should be recognized that a scored or roughened needle surface may complicate percutaneous insertion and/or subsequent passage of the needle to its desired position.
Examples of needles/stylets having scored or roughened outer surfaces include those described in U.S. Pat. No. 4,582,061 (Fry) as well as some of those described in the publication entitled "Laboratory Assessment of Ultrasonic Needle and Catheter Visualization" , by McGahan, John P., J. Ultrasound Med., (July 1986).
Apart from these prior efforts to improve the ultrasonic imageability of amniocentesis needles, other improvements have been developed with the intent of avoiding possible occlusion or clogging of the needle tip during aspiration. One such "improved" amniocentesis needle is disclosed in U.S. Pat. No. 4,308,875 (Young). Such improved needle comprises a hollow needle having a blunt round non-cutting tip with multiple communicating side holes formed in the distal 1 cm of the cannula. A solid stylet, having a sharpened distal tip, is positionable within the lumen of the needle such that the sharpened distal tip of the stylet protrudes beyond the distal tip of the needle. This protruding portion of the stylet provides the necessary cutting tip for penetration of the skin, fascia, and underlying structures. The blunt tip of the cannula disposed about the stylet does not cut and, therefore, must be thrust through the tissue as the cannula/stylet assembly is inserted. Following withdrawal of the stylet, fluid may be aspirated through the cannula. The purported advantage of the side holes is to facilitate uninterrupted withdrawal of amniotic fluid without obstruction or clogging.